1. Field of the Invention
The present invention generally relates to small propulsion systems for maneuvering spacecraft and, more particularly, is concerned with a hybrid electrothermal/electromagnetic arcjet thruster and thrust-producing method.
2. Description of the Prior Art
Arcjet Electrothermal (AJET) thrusters and magnetoplasmadynamic (MPD) thrusters are two types of electric propulsion mechanisms which utilize two different means of converting electrical energy to kinetic energy. These two different types of electric propulsion systems were actively researched during the early 1960's and thereafter generally ignored as requiring electric power far beyond that available to spaceships at that time and efforts were instead directed toward combustion type liquid and solid fuel rockets. MPD thruster are thought to be much more efficient that AJET thrusters, but require much more power. With recent advances in space ship electrical power sources, sufficient power is now possible to run small AJET thrusters but not MPD thruster on a spaceship or satellite. For an explanation from a historical perspective of the construction and operation of AJET and electromagnetic MPD types of arcjet thrusters, attention is directed to the following publications: "Physics of Electric Propulsion" by R. G. Jahn, McGraw-Hill Book Company, (hereinafter "Jahn") 1968, and "Electromagnetic Propulsion", McGraw-Hill Encyclopedia of Science and Technology, 1977, Vol. 4, pages 552-556.
Aside from the electric power requirements, there are inherent inefficiencies associated with each of these different propulsive mechanisms. In the AJET thruster, frozen flow losses associated with ionization and dissociation of the propellant waste a large portion of the input power leading to a low over all efficiency (less than 50%). In the MPD thruster, obtaining useful electromagnetic acceleration requires high current levels to convert the propellant into an electrically conductive moving plasma, that is to both substantially ionize the propellant and to accelerate the propellant ions. Typically the thermal energy of the propellant remaining from the ionization process is lost because of the poor expansion characteristic of MPD thruster designs. Upwards of a mega watt of power would be required to run an MPD steady state. In addition, a mega watt MPD steady state thruster would be many years of research away because of erosion and facility problems.
The arcjet as developed is well suited for geosynchronous and orbit transit missions, even with its low efficiency. For interplanetary missions and for larger spacecraft in or rear earth, however, a higher specific impulse, higher efficiency thruster would be needed. The MPD does not meet this need because of its development status and because of the very large power levels required. In view of this status, the availability of an alternative electric propulsion mechanism to either one of these two types which might produce substantially more efficiency and specific impulse than an AJET thruster but not require so much electrical power as an MPD would be highly desirable.